LED or other light-emitting elements are usually used as the light source for backlight in a liquid-crystal display device. A light source for backlighting usually has a light intensity adjusting function (light adjusting function) in order to adjust the visibility or increase the power consumed by lighting. The light adjusting function is usually realized by the drive circuit for the light-emitting element.
The simplest method of adjusting the amount of light emitted from an LED is to adjust the current flow to the LED. However, since the hue of the LED varies corresponding to the current, when the current is changed to adjust the amount of light, the hue of the LED also changes. In particular, in a white LED, the change in hue will significantly affect the display quality. Consequently, a PWM (pulse-width modulation) method is usually used to adjust the amount of light emitted by the LED.
In the PWM method, a periodic pulse current is supplied to the LED to flash the LED repeatedly. If the LED flashes very quickly, the brightness sensed by the human eye appears constant depending on the persistence. Since the on-time of the LED is adjusted by adjusting the pulse width of the current, the brightness sensed by the human eye changes. Since the current supplied to the LED by means of the PWM method can be held constant, the problem, that is, the change in hue along with the change in the amount of light, can be avoided.
The purpose of the present invention is to solve the problems by providing a drive circuit that can suppress the influence of the changes in temperature or power supply voltage and the variation in elements occurring during manufacturing and to output a pulse current with a constant level.